Welcome to Ikutozuki's Sona guide.
I hope to help other summoners interested in playing as Sona with a few tips and tricks to help you be the ultimate in being a support like Sona.

Why Sona you ask? Why not some better supports like Taric of Soraka?
Well, why not?
Her AOE buffs are amazing, her Ult makes everyone dance and to top it all off, she looks identical to Hatsune Miku. Yeah the last reason only will effect some people.

Pros:

Amazing Buffs
Great Support who can deal damage (with the right build)
Table-Turning Ult
Auto-Attack spells
Looks like Hatsune Miku
Doesn't scream "DEMACIA!!!!!" when you cast your ult

Cons:

Squishy, very very squishy
Early-game mana problems can screw you over
Magnet to ganks
Can be really bad when laned with the wrong champion
Doesn't scream "DEMACIA!!!!!" when you cast your ult

(Innate): After casting 3 spells, Sona's next attack deals extra 14 + (9 x level) magic damage.
Additionally, Sona's auras persist for 2 seconds after deactivating and set off a 2 second global cooldown. Power Chord has an additional effect depending on which spell was cast last.
The passive that makes me wanna burst into a guitar solo. Or not.
It is very important that you plan when this passive will activate. VERY IMPORTANT. It'll become less important as the game roles out but still important nonetheless.


(Stance) - Persistent Aura: Sona plays the Hymn of Valor, granting nearby allied champions bonus attack damage and ability power.
Activation: Sona sends out bolts of sound, dealing magic damage to the nearest two enemy champions or minions.
(Power Chord) - Staccato: If this spell was last cast when Sona's Power Chord is ready, her next attack will do double the damage of her Power Chord.
Your only true attack other than Crescendo.
It doesn't hit hard early game but it'll hurt against little dps Carries midgame. Then it'll water out lategame unless built with AP


(Stance) - Persistent Aura: Sona plays the Aria of Perseverance, granting nearby allied champions bonus armor and magic resistance.
Activation: Sona sends out healing melodies, healing herself and a nearby wounded ally.
(Power Chord) - Diminuendo: If this spell was last cast when Sona's Power Chord is ready, her next attack will de-buff the target to deal 20% less damage for 4 seconds.
The skill that you should max out first 90% of the time, unless under different circumstances. This should also be spammed in teamfights.


(Stance) - Persistent Aura: Sona plays the Song of Celerity, granting nearby allied champions bonus movement speed.
Activation: Sona energizes nearby allies with an additional burst of speed for 1.5 seconds.
(Power Chord) - Tempo: If this spell was last cast when Sona's Power Chord is ready, her next attack will slow the target by 40% for 2 seconds.
It makes you run faster. So when Sona tells you to run you better damn run.
This isn't a viable escape mechanism either. Be warned.


(Active): Sona plays her ultimate chord, forcing enemy champions to dance (stun) and take magic damage over 1.5 seconds.
When Sona tells you to dance. You better dance.
God this can change the whole aspect of a teamfight. Damage over time while dancing. Fun times.
ALSO: This shouldn't be used as an initiator unless it is the only one you have. It is best used in the middle of a teamfight right when your team is about halfway through their health of when the other team is backing off.

Oh so many plausibilities with runes for Sona.

Marks

:
Greater Mark of Magic Penetration
Pretty much the standard caster/support mark used by everyone. It's very useful for Hymn of Valor. Nothing else though. But it is one of the best options on Sona.

Seals

:
Greater Seal of Scaling Mana Regeneration
I love these things. Early game mana problems are eliminated so you can switch up your buffs without any worry (unless you're hammering your skill buttons)

Greater Seal of Replenishment
If you prefer to spam your skills early game, these work out really well.

Greater Seal of Knowledge
Since Riot buffed Sona on her mana costs, these help out well as well. Greater Seal of Scaling Mana Regeneration or Greater Seal of Replenishment help out much more though.

Glyphs

:
Greater Glyph of Cooldown Reduction
Oh cooldowns, I love this stat so much. With a full set of these babies and your masteries, you can easily reach 40.00% cooldowns with only Ionian Boots of Lucidity and Shurelya's Battlesong

Greater Glyph of Scaling Ability Power
Ability Power is viable, but you'll benefit from cooldowns more. Still, you'll pack a good punch mid game.

Quints

:
Greater Quintessence of Ability Power
Flat AP quints are fun. A good 15 AP at the beginning of the game and it can reach 23 with Hymn of Valor

Greater Quintessence of Health
Since Sona is very squishy, Flat health quints help her out.

Masteries

Sona follows a basic 9/0/21 mastery chart, picking up as much of CDR as she can. Make sure to grab the buff for the summoner spells you choose, ironically the most useful ones on Sona are in the Utility Tree.

The level sequence at the top is the basic Sona skill sequence that works the best if your laning with your carry.

Your Basic Sona

-One point in Hymn of Valor at level one
-Maxing out Aria of Perseverance first
-Leveling up Crescendo whenever you can
-Maxing out Hymn of Valor second
-Finish off Song of Celerity last

This allows you to keep your lane partner safe, keep you safe, and with Power Chord you can harass the opposing laners.
"So any other skill sequences work out?"

AP Burst Sona wrecks your face

-Max out Hymn of Valor first
-then Aria of Perseverance
-last Song of Celerity
- Crescendo whenever

This is a more risky skill sequence but man, look at all that damage you can throw out. Only do this when you're forced to lane with your tank.

Super Support!

- Aria of Perseverance first
- Song of Celerity second
- Hymn of Valor last and only when you're forced to level it up
- Crescendo whenever

Super Support Sona will save your life but can't hurt your enemies for her life. It's like an under appreciated Soraka. Do this skill sequence when you're laning with someone who can deal ALOT of damage. (a smart Jax, a smart Poppy, a smart KogMaw, etc.)

Most useful

The only escape method for Sona other than running away with Shurelya's Battlesong. If you can master flashing over walls and escaping through corners, you'll never die.

It allows you to spam your skills without worry. It also helps out your laning partner as well

Damn, this makes you unkillable. Having this spell gives so much map control and if you know where to place you spell with some teamwork, you can avoid the nastiest of ganks.

Viable

I've never actually used this all that much on Sona. It does help in teamfights if placed on their carry and can grant you kills but Sona's philosophy in games is Healing > Support kills > Kills

I prefer Flash over Ghost any day but it does come in handy if you refuse to get Shurelya's Battlesong, this speed boost combined with Song of Celerity, you'll run faster than a Master Yi on steroids.

It's not good, but it's not bad. Gets Sona places where she needs to be but other options are better.

The Nevers

Your not chasing kills and a built in slow with your Power Chord is all you'll need.

Okay, so it may be better early game compared to Aria of Perseverance but it won't replace it.

...really?

Only if your doing this Play Video
If you're tanking all the CC, you're doing something wrong.

Not that good on Sona. Let someone else of your team take it.

Hymn of Valor > Rally

Oohoohoo! This is where it gets fun. (Well fun for you who's not typing all of this.)

Basic Build for Sona

A sweet build for Sona. Lets you pull off face wrecking bursts and save your team from a horrible teamfight.
Start off by rushing Tear of the Goddess. Meki Pendant or Sapphire Crystal are both good choices for a starting item (along with a few potions). After Tear, grab either boots or start building Shurelya's Battlesong. After you achieve the holyness that is Shurelyas and those boots that totally match your dress, grab Sheen (or start building it if you don't have the cash.) Then finish it off by building Archangel's Staff and Lich Bane (Archangel > Lich so build Archangel first.)

Final Items

No matter how long a game lasted, I could never finish these items but man, they help so much. Deathcap is your second main AP item, and your last. Combined with Archangels makes you have over 400 AP. Plus the AP from Lich Bane makes your Power Chord and Lich proc. deal massive damage. Banshee's is a item that works good on anyone.

Other Great Items

Ooh look, another aura for Sona. This works great as a main health item. CDR and Mana Regen is fun too.

The "In case of emergency" item. It's passive lets you heal yourself more and the added MR is nice as well.

Another aura for Sona. Spell Vamp and AP means "hell yes" for your casters.

If your not rushing Tear, the your rushing Catalyst. Rod of Ages works wonders on Sona. She won't be squishy, and won't have mana issues early game.

I hate snowball items. I don't know why, I just despise them. I never get this item, even though you can reach a epic 20 stacks by just standing there. Still, I never get it. Doesn't mean can't or shouldn't. Actually it's preferred on Sona because of how easy you can get stacks.
Aegis of Legion
Yay for more auras. A good defensive item that can make Sona with [Aria of Perseverance] on.
Bonus attack damge is also nice too.

Out of the Gate.

Sona works best when dual laning. So pick your laning partner wisely. You don't want to lane with your tank because you won't be able to push or help said tank with heal. Laning with a good damage dealer is essential. Once you've chosen your lane partner, start spamming your skills and spawn until you have Power Chord up. It is a great harass and farm tool.
You want to play Sona very passively. Unless you can harass and get away with it, or someone else is doing it, just farm creeps until you can get Tear of the Goddess. Go back, grab it and a couple of pots and head back to lane. You may now spam your skills. With your runes, masteries and tear, you'll have amazing mana regen.

You and your laning partner

Sona is one of those supports who doesn't "babysit" as good as other supports. Sure you'll give all the experience to the carry but your overall usefulness will drop like bricks in teamfights. What should be done instead of babysitting is to stay in a proper range to get experience but leave all the last hitting to your partner.

Once your level 6

Yay your level 6 now. Level 6 comes with your ult and some wonderful choices.
You can either:
A: Continue to lane and eventually push the tower which leads to.
B: Run off, giving solo experience to your carry and you off to roam the other lanes.
Be warned that if you're going to roam be ABSOLUTELY sure that your carry can handle a 2 vs. 1 lane or that you have a jungler that can gank both bot lane and top lane when they can.
For more information about roaming, please read the Roaming section of this guide:
http://www.mobafire.com/league-of-legends/build/shield-n-heal-an-introduction-to-the-support-role-82908

Mid and Late Game Sona work in similiar ways.
You follow someone with low health, spamming your heal, then you follow someone else and spam them with your heals. That's great and all but what most Sona's shine at is when she enters a teamfight.

By the time your out of laning phase you'll be around the levels of 10 to 13. Your W skill will be mazed out and your on your way to max out your Q skill. The bonuses from these buffs will be amazing in teamfights. It lets your tank be tankier, your carry be more deadly and live longer than a housefly and your everything else more everything-ey...ness. (that's a word right?)
Now when you enter a teamfight be sure to NOT INITIATE WITH YOUR ULT UNLESS YOU HAVE TO. Crescendo's range is not that much and by the time you run in and cast it, you'll be dead. That's why you should always let the tank initiate. It is their job of course. What you should do with your ult is as soon as the initiate has initiated, wait till everyone is grouped up and cast it. This will allow either:
A: Your whole team to escape without dying (for the most part)
B: Stop an enemy team from tearing your carry to bits
C: Stop the enemy team from getting away from a losing teamfight.
It's kind of like a post-initiate.
Also, remember not to over commit in a teamfight. Your not their to stun everyone in your ult, you their to provide the carry with a soft pillow of love and chocolates. Stay in the back, spam your heal, and watch everyone die.

Sona is always overlooked as a support. She doesn't heal that well and can't wreck the squishiest of Teemo's. She always in the shadow of better supports like Janna or Taric but what she does have is great team support and the power to change the tide of a teamfight with just a spell cast. I really hope you enjoyed reading this guide and I'll update this as many times as humanly possible!

If you have any tips on how i can improve my guide, leave them in the comment section below!
Happy supporting!!